Method of making thermoelectric initiators of semiconductor material



March 28,- 1967 H. B FORNEY ETAL 3,311,635

METHOD OF MAKING THERMOELEGTRIC- INITIATORS OF SEMICONDUCTOR MATERIALOrlginal Filed May 5, 1962 INVENTORS HARRY B. FORNEY LLOYD E. LlNE,JR.

AT'TORNEYS United States Patent 3,311,685 METHDD OF MAKINGTHERMOELECTRIC INITI- ATORS OF SEMECONDUCTOR MATERIAL Harry B. Forneyand Lloyd E. Line, Jr., Richmond, and Carl S. Muller, Hanover County,Va., assignors to Texaco Experiment, Incorporated, Richmond, Va., acorporation of Virginia Original application May 3, 1962, Ser. No.192,272, now Patent No. 3,211,096. Divided and this application Jan. 8,1965, Ser. No. 430,787

1 Claim. (Cl. 264-111) This is a division of application Ser. No.192,272, filed May 3, 1962, now Patent 3,211,096.

This invention relates to a method for making thermoelectric firinginitiating devices and, more particularly, to a method of makingelectric initiators which are highly resistant to premature firing byalternating current including current induced by radio frequencyradiations.

The invention relates to a method of preparing electrical firinginitiating devices which generally include a casing in which is disposeda heater device in contact with a heat sensitive ignition composition ormatchhead, which, in turn, is embedded in or located adjacent to anexplosive charge.

Electrical firing initiating devices are commonly employed to initiatevarious explosive compositions used in conventional blasting caps, inordnance applications and as igniters for reaction motors of the liquid,gas or solid propellent types. In general, such initiators are designedto be actuated by direct current. The art has long recognized thedangers inherent in premature discharge of electric initiators byaccidentally induced alternating currents and this danger isparticularly acute in the application of such devices to space vehicleswhere radio frequency initiated guiding systems and control means areemployed together with electrical firing initiating devices.

It is, therefore, a primary object of this invention to provide a methodfor making a direct current initiated electric initiator safeguardedagainst premature or acci- 7 dental initiation by alternating currentsinduced therein.

A further object is to provide a method for making improved alternatingcurrent protected electric initiators without substantially reducing thedegree of sensitivity of the electric initiator to initiation by directcurrent of a predetermined polarity.

These and other objects and advantags are provided by an electricinitiator safeguarded against premature ignition by alternating currentinduced therein which generally comprises a pair of electricalconductors selectively conneotible at one end to a source of firinginitiating direct current of predetermined polarity, semiconductor meanshaving a P-N junction electrically connected to the pair of electricalconductors, a heat sensitive ignition composition in heat exchangerelationship to the P-N junction of the semiconductor means, and heatabsorbing means in contact with the semiconductor means and remote fromthe heat sensitive ignition composition and the P-N junction.

The invention will be more particularly described with reference to theillustrated embodiments thereof shown in the accompanying drawingswherein:

FIG. 1 is a schematic diagram of an electric initiator embodying theprinciples of the present invention;

FIG. 2 is a fragmentary diagrammatic view of apparatus suitable forconstruction of the semiconductors employed in the electrical initiatorillustrated in FIG. 1; and

ice

FIG. 2a is a view similar to FIG. 2, showing a modification.

It is known when direct current is passed through a junction ofdissimilar material such as P-type and N-type semiconductors, Peltierheating or cooling occurs at the junction depending on the direction ofcurrent flow. This heating or cooling is in addition to ordinary Jouleheating which also occurs regardless of the direction of current flow.When alternating current is passed through such a junction substantiallyonly Joule heating occurs.

If no Joule heating occurs at the junction of the dissimilar materialswhen alternating current is passed through the junction, an absolutelyalternating current proof electric initiator could be devised. However,Joule heating occurs thus requiring a modification in the design ofelectrical initiators employing semiconductor means to take into accountthe following requirements which oppose each other: the P-N junction ofthe semiconductor means which is in heat exchange relationship to a heatsensitive ignition composition, for many applications, must be broughtto the ignition temperature of the heat sensitive composition within afew milliseconds by the application of a reasonable direct current ofthe proper polarity; and the electrical initiator must toleraterelatively high alternating currents without developing suflicient Jouleheat to bring the heat sensitive ignition composition to its ignitiontemperature.

These requirements are satisfactorily met in a thermoelectric initiatorwherein the semiconductor means having the P-N junction is of smallcross-section and the free ends of the semiconductor means are connectedto heat absorbing masses or heat sinks.

The principles of the invention will be more readily apparent to thoseskilled in the art from the following detailed discussion of theinvention when referenced to FIG. 1 of the drawings, wherein 10generally designates an improved electric initiator constructed inaccordance with the teachings of the present invention. The electricinitiator 10 may include a pair of dissimilar materials 14 and 16interconnected along faces A and A to provide a junction, or thejunction may be provided in a single semiconductor crystal as is knownin the art.

The dissimilar materials may comprise P-type and N- type semiconductors14 and 16 maintained in contact with each other or connected together bya suitable electrical conductive connector 18. The opposite faces B andB of the semiconductors are in contact with heat absorbing masses 20 and22, the free ends of which are connected to electrical conductors 24 and26 which, in turn, are connected to a suitable source of properlypolarized direct current generally designated 28. Switch means generallydesignated 30 are provided in one of the conductors 24 or 26 or both forselectively connecting the initiator to the initiating source ofcurrent.

A sensitive initiator composition 32 is maintained in heat exchangerelationship to the P-N junction or the connector 18 where an electricalconductive connector is employed in the device. Further, the initiatorincludes a jacket or casing 34 which surrounds at least the junctionportion of the semiconductor means and maintains a suitable explosivecomposition 36 in contact with the heat sensitive initiator 32.

The junction forming materials 14 and 16 may com prise suitable P and Ndoped lead or bismuth tellurides,

manium, and the like. Semiconductors having high Peltier coefficientsare preferred and particularly good results have been obtained with thelead and bismuth tellurides.

Where the P-N junction is provided by connecting together P and N typesemiconductors and an electrical conductive material 18 is inserted atthe junction, the electrical conductive material may comprise aconductive silver epoxy adhesive, a bismuth-tin solder, a galliumcopperamalgam, a fine mesh metallic powder and the like.

The heat conductive members or heat sinks 20 and 22 may comprise anygood heat and electrical conductive material such as copper, aluminum,silver and the like.

The primer spot, matchhead, or heat sensitive initiator composition 32may comprise mercury fulminate or, for example, lead azide.

The casing 34 may be constructed of metal, plastic or other material andwhere an electrical conductive material is employed for the casing 34,the casing is insulated from the pair of electrical conductive heatsinks 20 and 22 by insulating means 38.

In the assembly of devices of the type described it is desirable tomaximize the ratio of the maximum tolerable steady state alternatingcurrent to the minimum direct cur rent for ignition of the explosivecomposition in, for example, milliseconds. In general, for a givenvoltage drop between faces B and B of the semiconductors, both thegeneration of Joule heat and the rate of heat conduction to the heatsinks and 22 vary inversely with the thickness of the semiconductors.However, for a predetermined current, the Peltier heat developed in theunit is generally independent of the thickness of the semi-conductorelements and the rate of heat conduction to the heat sinks 20 and 22during the transient period is always less than the heat conduction tothe sinks during the steady state condition. Therefore, it has beenfound desirable to maintain the semiconductors 14 and 16 relativelythin. Decreasing the thickness of the semiconductors has a substantialeffect on the magnitude of the alternating current required to produce agiven temperature rise at the junction since for a given temperature therate of heat conduction to the heat sinks 20 and 22 is inverselyproportional to the distance the heat must flow.

Semiconductor segments 14 and 16 of from about .01 to, for example, .08cm. in thickness and of about /1 inch to, for example, A inch indiameter have been found to provide very satisfactory results. Usingsemiconductors within this range, copper heat sinks 20 and 22 of aboutinch in length and from inch to about inch in diameter will provideprotection through a substantial range of frequencies where the requiredtemperature rise for initiation of the igniter is in the order of about200 C. and this temperature is to be developed by the application of apredetermined polarity direct current in not more than about 10milliseconds.

Example Commercial doped P- and N-type bismuth telluride semi-conductorstock was sliced into thin segments about .05 cm. in thickness and about.16 cm. in cross-sectional area. A pair of sliced segments were bondedtogether with a bismuth-tin solder and heat sinks were soldered to theopposite faces of the semi-conductors with each of the heat sinkscomprising a bar of copper .16 cm. in crosssection and about 0.6 cm. inlength. For test purposes a thermocouple was attached to the bismuth-tinsolder connection between the P- and N-junction of the semiconductorsand connected to recording apparatus. It was found that it requiredapproximately twice as much alternating current as polarized directcurrent to produce a steady state temperature rise of 200 C. aboveambient at the junction.

The device was found to require approximately 40 amp. of direct currentto raise the temperature at the junction 200 C. above ambient in .08second.

The device was tested employing as the alternating current source bothZ-megacycle and 60-cycle currents.

Slicing and surface finishing the semiconductor wafers for the initiatorwas found to be tedious and the thin segments were subject to fallingapart when the segmentation was perpendicular to the crystallinelaminates of the semiconductor stock. It was discovered thatsatisfactory semiconductors could be very conveniently produced bycompressing fragments of the semi-conductive material into thecross-sectional shape and thickness desired in the completed unit withno loss of the thermoelectric qualities of the semiconductor material.

Referring to FIG. 2, a method of producing compacted semiconductor unitsis diagrammatically illustrated. In FIG. 2 there is illustrated a press50 comprising a movable ram member 52 and a die block 54 having adepression 56 formed in a surface thereof a dimension corresponding tothe dimension of the desired semiconductor pellet. Into the depression56 is placed a fine granular doped semiconductor material 58, such aslead or bismuth telluride, and the material is compressed at pressuresin the order of at least about 50,000 p.s.i. and preferably about100,000 p.s.i. applied by conventional press means in the direction ofthe directional arrow. The compressed pellet was removed from thedepression 56 by applying pressure to the slide block 62, the topsurface of which forms the base of depression in the die block 54.

The semiconductor pellet may be formed about an electrical conductorillustrated at 60 during the formation of the compacted wafer. Where aconductor is desired in the semiconductor unit the conductor 60 may beled into the cavity or depression 56 through a bore in the slide block62. It is also contemplated that the heat sinks 20 and 22 may be formedwith undercut notches or grooves and that the fine granularsemiconductor material 58 may be compressed into the notches or groovesto provide a bond between the semiconductors and their heat sinks. Thisis shown in FIG. 20.

Example Fragments of doped bismuth telluride were placed in a depressioninch in diameter and approximately 0.3 cm. in depth. Enough of bismuthtelluride was placed in the cavity to provide a compacted semiconductorunit approximately .05 cm. in thickness and A inch in diameter. Thefragmentary semiconductor material was placed under compression at apressure in the order of 100,000 p.s.i. and the resulting pressed unitswere found to have better strength characteristics than the originalmaterial without apparent loss in thermoelectric characteristics.

From the foregoing description, it will be readily apparent to thoseskilled in the art that the present invention fully accomplishes theaims and objects he-reinabove set forth. Those skilled in the art willalso appreciate that modifications may be made in the disclosed form ofthe invention without departing from the scope of the appended claims.For example, the initiator illustrated in FIG. 1 may be further improvedby providing radio frequency shielding about the initiator to reduce theinduction of radio frequency energy to the semiconductors. Further, themethod of making the compacted semiconductor units may be applied to theformation of various shapes of semiconductors and for the formation ofP-N junctions between dissimilar semiconductor materials by compactingmultiple layers of fragments of N-type and P- type semiconductors into asingle element.

We claim:

The method of making a dense compacted semiconductor device formed of aheat sink electrode and a semiconductor wafer comprising the steps ofproviding a face of the heat sink electrode with undercut grooves,placing said electrode in a pressure die, placing a quantity of granularsemiconductor material on the side of said electrode which is grooved,and applying pressure to said 5 granular semiconductor material of atleast 50,000 pounds per square inch within said die, to thereby cornpactsaid semiconductor material and to also join the compacted semiconductormaterial wafer to the heat sink electrode.

References Cited by the Examiner UNITED STATES PATENTS 1,439,646 12/1922Smith 264-274 2,261,916 11/1941 Megow et a1 264-274 2,267,954 12/ 1941Schumacher 264-111 6 Sherwood 264-11 1 Veley 264-111 Haron 264-11 1Williams 264-111 OTHER REFERENCES The Compression of 46 Substances to50,000 kg./sq. cm., P. W. Bridgman, Amer. Acad. of Arts & Sciences, vol.74, No. 3, October 1940.

ROBERT F. WHITE, Primary Examiner.

I. R. HALL, Assistant Examiner.

